The Trap (part 2)

This one:

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Ignore the blue celadons for a second. Those are interesting too and we’ll get back to them in a minute. But the middle pot, a carbon trap shino that actually trapped carbon (as seen around the rim).

For many people this is a common occurrence. But for some reason the carbon trap shino at the Guild had not been trapping carbon. Since I’ve been with the Guild I’ve had it kinda, sorta, trap some carbon on the handle of a mug, once. And the rest of the time, nada. Just an orange glaze with a tendency to crawl.

The reason given for this used to be that ours is an updraft kiln and it’s “hard to get carbon trapping in an updraft kiln”.

You may have been following the trials and tribulations of me learning to fire the kiln. Well, Suzan Marczak who’s a firer in the Guild likes to say that the kiln ‘fires itself’. In a way, it can – it has a sweet spot,  a narrow range of parameters where you get good reduction, a good temperature climb, and a good firing. And the kiln hasn’t changed, so why the problems?

A bit of pottery geekdom:

To know what the temperature is, and how much reduction you’re getting, you can use a device called an oxyprobe. This gives a number (in millivolts) for temperature, and a number for the amount of carbon monoxide in the kiln.

The other way is to ‘sense’ the kiln, by listening to it, seeing the colour/size/behaviour of the flame coming out the chimney. And for temperature, use a pyrometer, and pyrometric cones, that bend after a given amount of heat work has been reached.

Our oxyprobe had been dying for months now. And now we got a new one. So we have a new toy but we don’t quite know how it works. Which means we go back to the cones-and-flame system and just record the numbers from the probe, to learn, again, what they actually mean.

Over the last few firings it turned out we had been going into reduction a little later than we thought. Not drastically so, because we were still getting the reds and celadons to work. This time we were going to go by the cone – we would look at the spy hole early and catch cone 010 on time, before it’s flat on the shelf.  And that gave me the idea to try and see if the shino would work this time.

Firing Fridays. Gotta love those. There I was, on a foggy, rainy day, before sunrise, lighting the kiln again. But this time there is nothing to relate. Not tales of heroic fights to get the kiln to move. It simply cruised through the firing without stalling. And we did catch cone 010 on time. Suzan told me to give it another inch of gas, and the kiln just went into reduction all on its own. And my test cup is the result – finally, in our kiln, shino as it should be. As long as you reduce early enough (OK. And put the pot in a good place for carbon trapping).

The rest of the day was unproductive as only a firing day (wake up at 6AM, go to check the kiln every 30 minutes) can be. But I did share my excitement about Thierry bowls (see https://pricklypotter.wordpress.com/2013/11/04/the-trap-part-1/) with Suzan, and so Friday was big flat bowl day. I made a single pot the entire day… But it was very nice. Ann (GolemDesigns) saw the bowl stiffening up on the table outside and took a picture of it, stark white against the rough table, in the dying November light:

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It’s not as flat as Thierry’s but the inside curve is perfect. Now who will fire such a huge monster in the gas kiln. How will I glaze it assuming it makes it…

The other interesting thing was the blue celadon test. During a recent online pottery geek discussion someone reiterated a piece of received knowledge about blue celadons – namely, that using potassium as a flux is better for the colour than using sodium. An online friend I have questioned this, and I decided to put it a small test. Design the same glaze twice – everything being the same except one will use as much sodium as possible, and the other potassium.

I made four cups – two of each. To be put in the kiln in pairs. And here they are:

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Sodium left, potassium right.

and the other pair:

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I leave it to you to judge which one is more blue. I found the results surprising, also because it seems the right one is more matte than the left one, in both pairs.

Another celadon. That worked only partly.

The part that kinda worked:

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The part that kinda didn’t:

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A side view:

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Even where it’s blue, it’s not the perfect blue that the other celadons are. This was placed on the edge of a tall, cut, uncapped shelf, opposite from the spy hole (open it – get oxygen in – less than perfect reduction). My fault for making it so tall. I made it to see how tall I can go by pinching a pot, adding a coil, pinching another section, and so on. Pretty tall it turned out, about a foot. But firing this tall guy (I think it’s a vase but I may be wrong) was never going to be easy. I console myself with the fact that at the Musée Guimet in Paris there is a celadon pot just like this: half reduced, half oxidized. The magic of atmospheric firing!

Another partner in crime on the tall shelf:

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Just the spout:

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Taking pictures of black pots is hard. I wanted to show the nearly metallic, matte surface. This pot has a friend from a previous firing:

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We had dinner guests two days ago, and one of them thought the handle on that was metal!

I like the shape of the second one better, but prefer the glaze of the first one. It also pours much better due to the slight flattening of the spout at the very end. The quest for perfection is never done.

And to finish, from the drama of tall pots to the comforting homeliness of cereal bowls… I had the hardest time glazing these – my glazes are usually black, or dark, or dramatic in some way. Who would want to eat cereal out of a black bowl? A celadon would not work since the shape of a cereal bowl has to be simple, dictated by its humble function. But celadon excels in accentuating an elegant form.

The solution:

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My turquoise glaze saves the day and provides a nice contrast to the yellow fake ash. The inside is another yellowish glaze, one that’s more durable than the quite soft fake ash:

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Eight of them, and they all worked. It’s good when things come out the way you want!

Edit – people have asked for pics where they can see the celadons better – I’ve taken new ones, in three different exposures. By the way, you can see a place where some copper flashed onto the Na celadon, turning the rim a bit pink! Na left, K right.IMG_8265 IMG_8266 IMG_8267

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3 thoughts on “The Trap (part 2)

  1. Robert

    Well isn’t that a fine to-do! Great celadon experiments, and despite the changes in feldspar they both actually look pretty nice. I certainly can’t see much of a difference in the “blueness” of them – especially if you take into account the differences in the nature of the glaze.

    I’ve watched Thierry Fouquet’s video a couple of time before, but I am unconvinced that my porcelain would withstand such abuse! Maybe I need to start with a thicker rim.

    That carbon spot chino tumbler came out pretty nicely too. All in all a great week’s work!

    Reply
  2. pricklypotter Post author

    Thanks Zsuzsi!

    Rob – I’ve posted new pics here. Also, porcelain, PVC pipe, try it out! it works for me. Thick rim, thick base, and more than that – a base wide enough to match the curve of the pipe.

    Reply

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